internal/task: rename tinygo_pause to tinygo_task_exit

This is more descriptive: the call is to exit a task, not to pause it.
This also makes it more obvious that there's an optimization
opportunity: to free the stack explicitly after the goroutine returns
(or to keep it as a cache for the next stack allocation).

src/internal/task/task_stack_386.S

@@ -24,7 +24,7 @@ tinygo_startTask:
     addl $4, %esp
 
     // After return, exit this goroutine. This is a tail call.
-    jmp tinygo_pause
+    jmp tinygo_task_exit
     .cfi_endproc
 
 .global tinygo_swapTask